Nanoelectonic devices based on nanowire networks

ABSTRACT

Semiconductor devices where networks of molecular nanowires (or nanofibers) are used as the semiconductor material. Field effect transistors are disclosed where networks of molecular nanowires are used to provide the electrical connection between the source and drain electrodes. The molecular nanowires have diameters of less than 500 nm and aspect ratios of at least 10. The molecular nanowires that are used to form the networks can be single element nanowires, Group III-V nanowires, Group II-VI nanowires, metal oxide nanowires, metal chalcogenide nanowires, ternary chalcogenide nanowires and conducting polymer nanowires.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor devices and the materials that are used as the semiconductor elements in such devices. More particularly, the invention is directed to the use of networks of molecular nanowires as the semiconductor element in such devices.

2. Description of Related Art

The publications and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and grouped in the appended bibliography.

Nanoscale electronic devices that include components other that silicon offer attractive alternatives to traditional devices made using photolithographic methods. Various wires, with dimensions less that one micron have been fabricated or grown and some of them have been demonstrated to function as active electronic devices and have been used as components of electronic devices.

For example, devices have been made which include Si and related nanowires and other semiconducting nanowires including those made from Group III-V or Group II-VI compounds (59). Transistor operation using individual nanowires has been demonstrated. However, fabrication of such devices, where a single nanowire connects the (source and drain) electrodes, are technically demanding, particularly at small dimensions.

Electronic devices that utilize carbon nanotubes have been demonstrated as possible replacements for fabricated devices (201). Diode, transistor and logic element operation has been demonstrated. Such devices are grown using high temperature conditions, and contain a mixture of semiconducting and metallic nanotubes.

Networks of nanotubes have also been shown to support Field Effect Transistor (FET) operation (202). However, doping and thus the tailoring of the conductivity of individual tubes is difficult. Accordingly, the optimization of device parameters is hampered.

In view of the above, there is a need for devices with the nanoscale conducting channels different from fabricated nanowires or nanotubes.

SUMMARY OF THE INVENTION

In accordance with the present invention, nanoelectronic semiconductor devices are provided where networks of molecular nanowires (or nanofibers) are used in place of the conventional semiconductor materials that are present in such devices. In a particular embodiment, field effect transistors are provided where networks of molecular nanowires are used to provide the electrical connection between the source electrode and drain electrode. The present invention not only covers the nanoelectronic devices themselves and the electronic devices they are used in, but also covers methods for making the nanoelectronic devices and methods for controlling the flow of electrical current between source and drain electrodes when networks of molecular nanowires are used as the semiconductor connection between the electrodes.

As a feature of the present invention, the molecular nanowires used to form the semiconductor networks have diameters of less than 500 nm and aspect ratios of at least 10. As another feature, the molecular nanowires that are used to form the networks can be single element nanowires, Group III-V nanowires, Group II-VI nanowires, metal oxide nanowires, metal chalcogenide nanowires, ternary chalcogenide nanowires and conducting polymer nanowires.

The present invention is applicable to a wide variety of electronic devices where networks of molecular nanowires (or nanofibers) can be used as conducting channels to satisfy the need for devices with the nanoscale conducting channels different from fabricated nanowires or nanotubes that was described above. Doped nanowires and/or nanofibers) also form part of the present invention. Furthermore, the molecular networks of nanowires in accordance with the present invention can be used in the fabrication of electronic devices, such as chemical or biosensors and as conducting elements for both passive and active electronic devices including resistors, diodes, transistors and logic elements.

The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a nanowire or nanofiber network based electronic device. In addition to a source (S) and drain (D), a gate (G) voltage can also be applied, in a usual transistor configuration, leading to a three terminal device.

FIG. 2 is a scanning electron microscopy image showing a polyaniline nanofiber network on a field effect transistor (FET) device. The average diameter of the nanofibers is 50 nm.

FIG. 3 is a graph showing the dependence of the source-drain current I_(sd) on the gate voltage V_(g). The dependence indicated p-type polyaniline doping. The hysteresis is common for liquid-gated devices. The dependence of the source-drain current on doping is due to the increasing conductance of the network.

FIG. 4 is a graph showing source-drain current (I_(sd)) versus source-drain voltage (V_(sd)). The linear behavior indicates that the current is determined by the polyaniline network. The dependence of the slope on the gate voltage V_(g) shows the transistor operation of the device.

FIG. 5 is a diagrammatic representation of the formation of nanowire networks in accordance with the present invention using chemical vapor deposition (CVD).

DETAILED DESCRIPTION OF THE INVENTION

An exemplary field effect transistor (FET) that utilizes a network of molecular nanowires (or nanofibers) in accordance with the present invention is shown generally in FIG. 1 at 10. The FET 10 includes a source electrode (S) 12, a drain electrode (D) 14 and a gate electrode (G) 16. The network of molecular nanowires is shown at 18. As is typical in any FET, an electrically insulating layer 20 is provided between the gate electrode and the semiconductor material (nanowire network 18). The insulating layer can be silicon dioxide (see Ref. 202), non-conducting polymer, such as epoxy. The electrodes can be made from any of the materials used in conventional FET devices. The FET operates in the same manner as conventional FET's except that that typical semiconductor material that is present between the source and drain electrodes is replaced with a network of molecular nanowires.

For the purposes of this specification, molecular nanowires are defined as having dimensions less than 500 nm in diameter (the diameter is the average of the cross-sectional width) and have an aspect ratio exceeding 10 (e.g. a 100 nm diameter nanowire must have a length that is equal to or greater than 1 micron). The term “molecular nanowire” is used herein interchangeably with “molecular nanofibers” and it is intended that when the term “molecular nanowire” is used alone, it includes molecular nanofibers.

The network of molecular nanofibers 18 can be made from a variety of known molecular semiconductor nanowires. Set forth below is a listing of known exemplary molecular nanowire materials that can be used to make networks of molecular nanowires in accordance with the present invention.

Single element nanowires made from silicon using known procedures may be used to form the network 18. The procedures for making such nanowires are set forth in detail in Refs. 1-21. Single element nanowires made from germanium may also be used. Details of synthesis are set forth in Refs. 9, 17 and 22-27. Other exemplary single element nanowires include selenium and tellurium nanowires, which are made according to known procedures as set forth in Refs. 28-29 and Ref. 30, respectively.

Nanowires made from a combination of Group III-V materials using known procedures may be used to form the network 18. Exemplary Group III-V materials that can be used to form nanowire networks include Ga, In, N, P, As and Sb. Details of exemplary synthesis procedures for these nanowires are set forth as follows: GaN (Refs. 8, 31-45); GaP (Refs. 39, 46 and 47); GaAs (Refs. 42 and 48-50); InN (Ref. 51); InP (Refs. 8, 38 and 52-54); and InAs (Ref. 55).

Nanowires made from a combination of Group II-VI materials using known procedures may also be used to form the network 18. Exemplary group II-VI materials that can be used to form nanowire networks include Zn, Cd, Hg, S, Se and Te. Details of exemplary synthesis procedures for these nanowires are set forth as follows: ZnS (Refs. 56-60); ZnSe (Refs. 44, 59 and 60); CdS (Refs. 59-72); CdSe (Refs. 59, 60, 65, 68, 69, 71 and 73); CdTe (Refs. 65, 73 and 74); and HgS (Ref. 75).

Nanowires made from metal oxides using known procedures may be used to form the network 18. Exemplary metal oxide nanowires and references to the details for making them are as follows; CdO (Refs. 76-78); Ga₂O₃ (Refs. 79-88); In₂O₃ (Refs. 85 and 89-99); MnO (Refs. 100-102); NiO (Ref. 103); PbO (Ref. 104); Sb₂O₃ (Ref. 25); SnO₂ (94 and 105-112); and ZnO (Refs. 113-117).

Nanowires made from metal chalcogenides using known procedure may be used to form the network 18. Exemplary metal chalcogenides that can be used to make nanowires include Mn, Fe, Co, Ni, Cu, Ag, Sn, Pb and Bi. Exemplary metal chalcogenide nanowires and references to the details for making them are as follows: Ag_(x)M_(y) (Refs. 29 and 118-124); Bi_(x)M_(y) (Refs. 125-134, 135 and 136-137); Co_(x)M_(y) (Ref. 138); Cu_(x)M_(y) (Refs. 139 and 140); MnM (Ref. 141); NiM₂ (Ref. 142); PbM (Refs. 114 and 143-152); and 5 nM (Refs. 153 and 154). M is Se, S or Te. The numbers for x and y are known in the art and typically will range from 1 to 9.

Nanowires made from ternary chalcogenides using known procedures may also be used to form the network 18. Exemplary ternary chalcogenide nanowires and references to the details for making them are as follows: CuInM (Ref. 155); AgSnM (Ref 156); CdMnM (Ref. 141); and CdZnM (Ref 157) where M also can be Se, S or Te.

Nanowires (also referred to as nanofibers) made from conducting polymers may be used to form network 18. Exemplary conducting polymer nanowires and references to the details for making them are as follows: polyaniline (Refs. 82 and 158-167); polypyrrole (Refs. 158, 160 and 168-170); and polythiophene (Refs. 158, 169 and 171-173).

The exemplary nanowires described above are deposited on the insulating layer 20 using any of the known techniques for forming a network of nanowires on a surface. Exemplary deposition methods that can be used to form nanowire networks on substrates include the following:

1. Solution Casting:

A great variety of nanowires can be made in solution and cast onto a substrate. See Refs. 28, 29, 50, 64, 68, 75, 96, 126, 131, 140, 143, 153 and 174-194 for details of exemplary procedures that may be used to make solutions of nanowires. These nanowires can be readily deposited onto an FET device by drop casting. Upon drying the solvent, network structures form. For example, we deposited a polyaniline nanowire network on a silicon wafer cast from a water dispersion using the procedure described in detail in Ref. 164. A scanning electron microscopy image of the resulting polyaniline nanowire network is shown in FIG. 2.

2. Langmuir-Blodgett Techniques:

Nanowires self-assemble into interconnecting networks when organic solvents containing nanowires are spread onto a water surface. The network can then be transferred from the water surface to a solid substrate by Langmuir-Blodgett techniques. Details of such procedures are set forth in Refs. 195-197.

3. Direct Growth of Nanowires by Chemical Vapor Deposition (CVD):

Using chemical vapor deposition, some nanowires can be directly grown as networks on substrates as diagrammatically shown in FIG. 5. Details of an exemplary CVD procedure for forming a network of nanowires as set forth in Ref. 198.

4. Electrospinning.

In a similar fashion to spider web networks, electrospining has been demonstrated to form networks of polymer nanowires/fibers on solid substrates (see Refs. 199 and 200). In a typical process, a polymeric melt or solution is extruded from the orifice of a needle to form a small droplet. In the presence of a strong electric field, charges built up on the surface of the droplet will overcome the surface tension to induce the formation of a liquid jet that is subsequently accelerated toward a grounded target. As the solvent is evaporating, this liquid jet is stretched to many times its original length to produce nanofibers (nanowires) of the polymer. The nanofibers are collected as inter-weaving networks on a spinning target.

Networks in accordance with the present invention are composed of an interconnected collection of two or more one-dimensional nanostructures (including nanowires/rods/fibers/tubes and the like) that establish at least one continuous pathway from the source 12 to drain 14. The nanowires may include dopants, if desired. Exemplary dopants for polyaniline include any Bronsted or Lewis acid, such as described in U.S. Pat. No. 5,096,586 issued Mar. 17, 1992. See especially from line 21 in column 8 to line 14 in column 9.

Examples of practice are as follows:

Polyaniline nanofiber networks were prepared using the same procedure as set forth in Ref. 164 and used to make nanoscale transistors of the type shown in FIG. 1. The films of polyaniline nanofiber networks were deposited on an epoxy insulated packaged silicon die with source and drain contacts. Gating was provided by an electrode and the network covered with a liquid (water). This type of gating configuration has been used for carbon nanotube based devices for the demonstration of transistor operation (See Ref. 202).

The dependence of the source-drain current I_(sd) on the source-drain voltage V_(sd), (I_(sd)-V_(sd)) together with the dependence of I_(sd) on the gate voltage I_(g) (the I_(sd)-V_(g) characteristics) were measured. The device was exposed to 100 mM HCl, and subsequently the I_(sd)-V_(g) curves were measured. In order to avoid strong gate to drain current leakage the gate frequency was kept below 0.1 Hz, V_(g) amplitude around 0.3V. The concentration of HCl was chosen to have a detectable source—drain current at low (V_(sd)≦50 mV) bias. With these parameters the leak current was observed to be below 20 nA.

In the first one hour after the application of HCl the I-V_(g) curves “shifted upwards” as shown on FIG. 3. In FIG. 3, the curve labeled “a” was obtained right after deposition of the HCl. The curve labeled “b” was obtained at 45 minutes after deposition. The curves labeled “c” and “d” were obtained at 60 and 75 minutes after deposition, respectively. The behavior as shown in FIG. 3 is expected and is due to HCl doping of the polyaniline fibers. When the response stabilized, I_(sd) (V_(sd)) curves were taken at several fixed gate voltages and the results are shown in FIG. 4. The dependence of the source-drain conductance again is consistent with a transistor operation with a p-type conducting channel. As a consistency check, HCl was replaced with an ammonia solution. The ammonia solution removes the dopants and leads to an insulating network, with resistance changing from 100 kΩ to 1 GΩ.

The preceding example showing the use of a network of molecular nanowires in accordance with the present invention demonstrates: field effect transistor (FET) operation; establishes a route for device optimization through doping; and further establishes that these devices can be used as sensors in a liquid environment.

Some of the advantages of semiconductor devices that utilize the network of molecular nanowires in accordance with the present invention include: 1) simple fabrication of nanowires or nanofibers, no need for CVD growth; 2) selected quasi one-dimensional conduction path; 3) chemically tunable conducting properties; and 4) such wires are robust and flexible, allowing integration onto flexible surfaces.

The advantages provided by using networks of molecular nanowires include: 1) many nanowires act as the conducting element, statistical averaging will occur, strongly reducing the signal variation from device to device; 2) by virtue of the large number of molecular nanowires involved, the structure is also “defect tolerant”. In case of sensor applications, the advantages include: 1) the surface area is large; 2) the conducting path is sensitive to environmental changes; and 3) the size of the nanowires is compatible with biomolecules.

Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the above preferred embodiments and examples, but is only limited by the following claims.

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1. A semiconductor device comprising: a drain electrode; a source electrode; a gate electrode; a semiconductor layer located between said drain electrode and said source electrode, said semiconductor layer providing an electrical connection between said drain electrode and said source electrode wherein said semiconductor layer comprises a network of molecular nanowires; and a layer of electrically insulating material located between said gate electrode and said semiconductor layer.
 2. A semiconductor device according to claim 1 wherein said network of molecular nanowires comprises nanowires having diameters of less than 500 nm and an aspect ratio of at least
 10. 3. A semiconductor device according to claim 1 wherein said network of molecular nanowires comprises molecular nanowires selected from the group consisting of single element nanowires, Group III-V nanowire, Group II-VI nanowires, metal oxide nanowires, metal chalcogenide nanowires, ternary chalcogenide nanowires and conducting polymer nanowires.
 4. A semiconductor device according to claim 3 wherein said network of molecular nanowires comprises one or more conducting polymer nanowires.
 5. A semiconductor device according to claim 4 wherein said one or more conducting polymer nanowires consist essentially of polyaniline.
 6. In a field effect transistor that includes a source electrode, which is electrically connected to a drain electrode by way of a semiconductor material, wherein the improvement comprises using a network of molecular nanowires as said semiconductor material.
 7. The improvement in field effect transistors according to claim 6 wherein said network of molecular nanowires comprises nanowires having diameters of less than 500 nm and an aspect ratio of at least
 10. 8. The improvement in field effect transistors according to claim 6 wherein said network of molecular nanowires comprises molecular nanowires selected from the group consisting of single element nanowires, Group III-V nanowire, Group II-VI nanowires, metal oxide nanowires, metal chalcogenide nanowires, ternary chalcogenide nanowires and conducting polymer nanowires.
 9. The improvement in field effect transistors according to claim 8 wherein said network of molecular nanowires comprises one or more conducting polymer nanowires.
 10. The improvement in field effect transistors according to claim 9 wherein said one or more conducting polymer nanowires consist essentially of polyaniline.
 11. A method for making a semiconductor device comprising the steps of: providing a drain electrode; providing a source electrode; providing a gate electrode; providing a semiconductor layer located between said drain electrode and said source electrode such that an electrical connection between said drain electrode and said source electrode is formed wherein said semiconductor layer comprises a network of molecular nanowires; and providing a layer of electrically insulating material located between said gate electrode and said semiconductor layer.
 12. A method for making a semiconductor device according to claim 11 wherein said network of molecular nanowires comprises nanowires having diameters of less than 500 nm and an aspect ration of at least
 10. 13. A method for making a semiconductor device according to claim 11 wherein said network of molecular nanowires comprises molecular nanowires selected from the group consisting of single element nanowires, Group III-V nanowire, Group II-VI nanowires, metal oxide nanowires, metal chalcogenide nanowires, ternary chalcogenide nanowires and conducting polymer nanowires.
 14. A method for making a semiconductor device according to claim 13 wherein said network of molecular nanowires comprises one or more conducting polymer nanowires.
 15. A method for making a semiconductor device according to claim 14 wherein said one or more conducting polymer nanowires consist essentially of polyaniline
 16. A method for making a semiconductor device according to claim 11 wherein said step of providing said semiconductor layer comprises forming said network of molecular nanowires on said layer of electrically insulating material by solution casting, a Langmuir-Blodgett technique, chemical vapor deposition or electrospinning.
 17. A method for controlling the flow of electrical current between a source electrode and a drain electrode in a field effect transistor, said method comprising the steps of: 1) providing a field effect transistor comprising: a drain electrode; a source electrode; a gate electrode; a semiconductor layer located between said drain electrode and said source electrode, said semiconductor layer providing an electrical connection between said drain electrode and said source electrode wherein said semiconductor layer comprises a network of molecular nanowires; a layer of electrically insulating material located between said gate electrode and said semiconductor layer; and 2) applying an electrical potential to said gate electrode to thereby provide control of the flow electrical current between said source electrode and said drain electrode.
 18. A method for controlling the flow of electrical current between a source electrode and a drain electrode in a field effect transistor according to claim 17 wherein said network of molecular nanowires comprises nanowires having diameters of less than 500 nm and an aspect ratio of at least
 10. 19. A method for controlling the flow of electrical current between a source electrode and a drain electrode in a field effect transistor according to claim 17 wherein said network of molecular nanowires comprises molecular nanowires selected from the group consisting of single element nanowires, Group III-V nanowire, Group II-VI nanowires, metal oxide nanowires, metal chalcogenide nanowires, ternary chalcogenide nanowires and conducting polymer nanowires.
 20. A method for controlling the flow of electrical current between a source electrode and a drain electrode in a field effect transistor according to claim 19 wherein said network of molecular nanowires comprises one or more conducting polymer nanowires.
 21. A method for controlling the flow of electrical current between a source electrode and a drain electrode in a field effect transistor according to claim 20 wherein said one or more conducting polymer nanowires consist essentially of polyaniline. 